The present invention relates generally to weight measurement methods and systems, and more specifically, to weight measurement methods and systems for material in a dynamic vacuum material transfer or other system utilizing differential conveying fluid pressure.
Shown in FIG. 1 is an example of a dynamic vacuum bulk material transfer system. Before the batching tank 110 is evacuated, the system operator zeros a weight measurement device 120 that measures the weight of the batching tank 110. A vacuum pump 130 is attached to batching tank 110 and activated to create a vacuum. After the tank is suitably evacuated, the operator causes material from one or more of the storage tanks 1051 . . . N to be delivered to batching tank 110 by opening a corresponding one of the valves 1251 . . . N. The vacuum pump 130 creates a pressure difference in the system. Materials are transferred from one of the storage tanks 1051 . . . N to the batching tank 110 by air currents caused by the pressure difference in the system. After the weight of the batching tank 110 has reached a desired weight the operator will cause the flow of material from the one of the storage tanks 1051 . . . N to cease by closing the corresponding one of the valves 1251 . . . N. The operator may continue to add materials to the batching tank 110 from the storage tanks 1051 . . . N, delivering a desired amount of each material to the batching tank. Thereafter, the operator will shut off the vacuum pump 130. The operator will also cause the material in the batching tank 110 to be transferred to the destination vessel 115.
The method described above may cause an inaccurate amount of material to be delivered to the batching tank 110. This is because the weight measurement device 120 is zeroed before the tank is evacuated. While the batching tank 110 is evacuated, air is removed, lowering the weight of the batching tank 110. As material is transferred to the batching tank 110, the material fills some volume. As this volume transferred approaches the volume of the batching tank 110, the weight of the batching tank 110 and the material in the batching tank 110 approach their weight at atmospheric pressure. During filling, however, the weight reported by the weight measurement device 120 is the weight of the material in the vacuum, and not the weight of the material at atmospheric pressure. Therefore, there is a discrepancy in the measured weight of the material during dynamic vacuum material transfer and the measured weight of the material at atmospheric pressure.
When the operator fills the batching tank 110 with material, he/she will fill the tank until the scale reads the amount of material that is desired in the tank. The reading of the weight measurement device, however, reflects not only the weight of the material in the tank, but also the difference between the amount of air initially in the tank and the amount of air in the evacuated tank. If the batching tank 110 has a large volume, the vacuum may decrease the weight on the scale dramatically. This process may lead to inaccurate measurement of material in the batching tank 110. For example, oilfield cement is mixed in a large batching tank 110, so that the weight difference between an evacuated batching tank 110 and unevacuated one may amount to hundreds of pounds.
One way to obtaining the weight of the batching tank 110 at atmospheric pressure involves venting the tank. The batching tank 110 may be vented after each material is added to determine the weight of the batching tank 110 at atmospheric pressure. A drawback of this approach is that significant time is spent venting and then re-evacuating the tank.
Another way of obtaining a more accurate weight of the batching tank 110 involves the use of a flow meter. If a flow meter is inserted between the storage tanks 1051 . . . N and the batching tank 110, the volume of material transferred can be measured. If the density of the material is known, the weight of the material transferred to the batching tank 110 can be calculated. However, in dry material transfer, a flow meter will tend to either block the passage between the storage tanks 1051 . . . N and the batching tank 110, or the flow meter may be destroyed by the material while it passes though the flow meter.